I think what a modern recon unit really needs to gain to work well is not increased movement and sight but rather some kind of stealth.
It seems to me by the time explorers come on line the entire world is not only explored, but barring a special map type is pretty much all settled, which makes them not really explorers at all.
I'd even say that recon units in the early game are a fundamentally different unit type than recon units in the late game, so there's no particular reason for one to progress into the other. Early game recon units are explorers, late game recon units are more like spies.
I'm strongly against stealth. I just don't see it working for land units. But if you have a specific suggestion, convince me

I disagree that most of the map is explored when explorers come around. This will only be the case with small pangea-like maps. Or perhaps larger pangea maps if you invested heavily in scouts. On most maps there will be entire continents and such that you have not be able to reach untill that moment. And on standard and larger pangeas there will likely still be undiscovered parts.
I don't think recon units should be spies. Though their high sight range later on will allow you to look deeply in enemy territory, but they will not go unnoticed.